1. Field of the Invention
This invention relates to a photovoltaic solar water heater system. More particularly, this invention relates to a water heating system wherein the load characteristics of an electric water heater are matched to the power generation characteristics of a photovoltaic array for maximum efficiency in water heating using solar energy.
2. Description of the Prior Art
There has been for many years substantial interest in turning the energy of the sun into useful power for various purposes, including the heating of water for residential use. To date, solar water heaters have heated the water directly by exposure to solar radiation, for example, by pumping the water to be heated through solar collector panels. Such systems involve relatively complicated plumbing in relatively inconvenient locations, increasing initial cost and maintenance expense.
The art also teaches subjecting a photovoltaic array to solar radiation for generating electrical power for a variety of purposes, including powering conventional appliances. As such appliances are normally powered by AC power from the utility grid, while photovoltaic arrays produce DC, such systems require the use of a complex power inverter to convert the DC voltage provided by the photovoltaic array to the AC voltage provided by the electric utility. Other systems in which the photovoltaic array is not interconnected to the utility grid require the use of storage batteries to power various devices during periods of inadequate solar radiation. In particular, the art fails to teach a system in which the inherent electrical generating characteristics of the resistive load provided by an electric water heater is matched to the electrical generating characteristics of a photovoltaic array.
The art includes numerous patents directed to photovoltaic systems for generating electric power from the energy of the sun. U.S. Pat. No. 3,384,806 to Hartman teaches a system wherein a photovoltaic array is connected to a particular load by an impedance matching device. Hartman does not teach doing so specifically to power an electric hot water heater. Hartman's device includes a DC-to-DC converter which would be relatively inefficient and undesirably complicated.
U.S. Pat. No. 3,696,286 to Ule recognizes that the power generation characteristics of a particular photovoltaic array differ with varying ambient conditions, e.g., temperature and intensity of solar radiation. Ule teaches determining the voltage at which maximum power would be produced by a photovoltaic array through the use of a reference solar array, and operating the device in conjunction with storage battery so as to achieve maximum efficiency. Ule does not particularly discuss heating water using electrical power generated with a photovoltaic array.
U.S. Pat. No. 4,175,249 to Gruber teaches reconfiguring a photovoltaic array to optimize the power characteristics of the photovoltaic array in accordance with the intensity of incident solar radiation. Gruber does not particularly teach doing so in connection with powering electric water heaters.
U.S. Pat. No. 4,314,198 to Rogers teaches a solar power source for a lighting device involving a battery to be charged during peak sunlight hours and discharged during darker evening and night hours. The use of a battery to store energy is an undesired complexity in an electric water heating system.
U.S. Pat. No. 4,390,940 to Corbefin et al teaches modulation of DC power obtained from a photovoltaic array to AC and varying the rate of modulation to obtain maximum power output. DC to AC conversion (with or without varying the rate of modulation) would be an undesired complexity in any water heating system.
U.S. Pat. No. 4,404,472 to Steigerwald teaches measuring the output voltage of a photovoltaic array and withdrawing current proportional to the voltage using a variable gain device. The variable gain device is typically a DC to AC inverter in the case of an utility load, or a DC to DC converter if the load is a battery. Steigerwald does not particularly address the question of heating water using solar energy. The use of inverters and converters is undesirably complex in such a system.
U.S. Pat. No. 4,649,334 to Nakajima also teaches controllable variation in the operating parameters of a power converter connected to a photovoltaic array to obtain maximum power output. Again, such a converter is undesirably complex and in particular is not appropriate in conjunction with a resistive load such as a water heater.
U.S. Pat. No. 4,916,382 to Kent also teaches use of a converter for converting power generated by a photovoltaic array to electrical power having different voltage and current characteristics. Again, use of a power converter is an undesired complexity not appropriate for driving a purely resistive load such as a water heater.
Finally, U.S. Pat. No. 3,666,918 to Clark, Jr. et al teaches certain improvements in conventional electric water heaters.
Thus it can be appreciated that the art as discussed above does not directly address the optimization of a system for heating water using electric power generated by a photovoltaic array exposed to incident radiation from the sun.